1. Field of the Invention
The present invention relates to the field of chopper-stabilized operational amplifiers.
2. Prior Art
Operational amplifiers (op amps) are high gain amplifiers typically having a differential input and a single ended or differential output. They are commonly coupled with negative feedback, so that the gain and frequency response of the overall circuit is determined primarily by the combination of the input circuitry and the feedback circuitry, not by the gain and frequency response of the op amp itself, at least at frequencies wherein the gain of the op amp remains high. A perfect op amp will have a zero output when the differential input is zero, and an infinite gain and zero phase shift, at least throughout the frequency range of interest. However a real op amp will have a high but finite gain, which typically will decrease at high frequencies, and some phase shift at higher frequencies characteristic of the particular op amp design. Also, a real op amp will not have a zero output for a zero differential input, but rather will have a zero output at some finite differential input. In a closed loop system, the system will settle at a differential input that will provide the required op amp output. Since the gain of the op amp is high, this differential input is substantially equal to the differential input that will cause a zero output. This is commonly referred to as the input offset, and will cause an error in the output of the system equal to the input offset times the gain between that input and system output.
Now referring to FIG. 1, a block diagram of a prior art amplifier consisting of a differential output stage A2 driven by a differential input stage A1 may be seen. Imperfect component matching inside input stage A1 results in an input offset voltage Vos1 which can be modeled by a small voltage source Vos1 in series with the input terminals of input stage A1, which stage may then be considered offset free (differential output stage A2 may also have some input offset, but its effect is divided down by the gain of the differential input stage, so may have little effect on the final amplifier output). Voltage feedback around the amplifier in a feedback system will force the voltages on the input terminals IN+ and INxe2x88x92 of input stage A1 to be substantially equal, resulting in a voltage difference Vos1 between the operational amplifier""s input terminals INP and INM.
FIG. 2a is a block diagram illustrating a well known method of reducing the offset, namely by adding so-called choppers S1 and S2 before and after input stage A1. The choppers consist of switches with two positions. In the first position, the inputs I1 and I2 are connected to the outputs O1 and O2, respectively. In the second position, the inputs I1 and I2 are connected to the outputs O2 and O1, respectively. The choppers S1 and S2 are synchronized to repeatedly switch between the first and the second positions at the rate of a clock signal f1. This action alternately inverts the effect of the offset voltage Vos1, and twice inverts the effect of the differential input INPxe2x88x92INM. The alternate double inversion of the differential input has no effect on the output OUTPxe2x88x92OUTM. However the alternate inversion of the effect of the offset voltage converts the offset voltage Vos1 into a square wave signal between the input terminals INP and INM with a peak-to-peak voltage amplitude equal to 2*Vos1 and an average value of zero. An amplifier that achieves high precision (low apparent input offset) in this manner is referred to herein and in the art as a chopper amplifier.
An additional advantage of the chopping technique is that it eliminates the so-called 1/f or flicker noise. This type of noise is present in all active devices and can easily be the dominating error source in amplifiers consisting of MOS transistors. Because 1/f noise is mostly present at low frequencies, it can be modeled as a slowly changing offset voltage. For the purpose of this invention disclosure, no distinction will be made between 1/f noise and offset voltages. The chopping action modulates the offset up to the chopping frequency, and even though the average value of the offset is now zero, filtering is required to remove frequency components at and above the chopping frequency from the signal bandwidth. This effectively reduces the usable signal bandwidth of the chopper amplifier.
Another prior art amplifier is shown in the block diagram of FIG. 2b. Here the amplifier signal path is through amplifier stages A1 and A2. Choppers S3 and S4 and amplifier stage Gm3 form a chopper amplifier having an output proportional to the input offset of the amplifier formed by amplifier stages A1 and A2, plus an AC component at the chopping frequency. Amplifier Gm4 together with feedback capacitors form an integrator, filtering out the AC component at the chopping frequency, and integrating the output of the chopper amplifier to apply an offset trim signal to amplifier stage A1 to minimize the input offset of the amplifier formed by amplifier stages A1 and A2. Amplifiers using a chopper amplifier out of the main amplifier signal path to provide an input offset trim signal to an amplifier stage in the main signal path are referred to herein and in the art as chopper stabilized amplifiers, with the trim accomplished by the use of a chopper amplifier being referred to herein and in the art as chopper stabilization.
An amplifier in accordance with the present invention achieves very high precision in combination with good high-frequency behavior. High precision, in this context, refers to an amplifier having a very low input offset voltage, where the input offset voltage is defined as the voltage difference between the input terminals necessary to drive the single ended output terminal to zero, or to drive the differential output to zero.
An amplifier topology is described that combines the use of chopping and chopper stabilization, and may include auto-zeroing to achieve this very low offset. The use of choppers guarantees a very low offset, and moreover, eliminates the so-called 1/f noise. Chopper stabilization is used to avoid the so-called chopper noise associated with chopper amplifiers. The auto-zeroing technique may be used to improve the performance of the chopper-stabilization circuitry. Various exemplary embodiments are disclosed.